Work in the present period studied the determinants of protein hydrogen exchange (HX) and employed HX capabilities and other methods to study global and subglobal stability, structure change, and protein folding. Systems studied were cytochrome c (Cyt c), ribonuclease A, a hyperthermophilic rubredoxin, the GroE system, and whole active muscle. Some major advances were made. A model proposed for the protein folding process provides a coherent explanation for available information on the equilibrium and the kinetic steps in protein folding in terms of a classical more or less sequential pathway of distinct intermediates and defined barriers, proceeding within the developing native-like context. A new 'stability labeling' method to test for the on-pathway nature of intermediates was successfully applied to the folding intermediates previously found in Cyt c. A facile high yield recombinant Cyt c system was produced and used to generate mutants designed to explore the folding process. This system will also be useful for a manifold of other Cyt c studies. A forceful energy-dependent unfolding of a substrate protein by the GroE system was demonstrated. Other goals were accomplished. Initiatives proposed will build on these advances. A histidine scanning series will be used to kinetically trap and populate the different intermediates in Cyt c folding; the intermediates can then be separately identified and their structural and kinetic relationships established. A glycine scanning series will be used in the equilibrium stability labeling mode to test for the on-pathway nature of Cyt c folding intermediates and to help establish this approach for other proteins more generally. The glycine mutants will also be used to characterize the previously enigmatic 'local fluctuations' that determine the HX behavior of most protein hydrogens under most conditions. An approach will be attempted that promises to make the native state HX method generally applicable to the majority of proteins which have so far not been accessible to this analysis. Other work is planned.